1. Field of the Invention
The present invention generally relates to a fluid flow control valve assembly and, more particularly, to a throttle valve assembly for controlling the flow of a fluid medium therethrough.
2. Description of the Prior Art
It is well known that a fluid circuit used in various applications include one or more fluid flow control valve assemblies each operable to regulate the flow of a fluid medium under pressure. A throttle valve assembly is an example of the fluid flow control valve assembly.
The prior art throttle valve assembly generally comprises a housing having defined therein an inflow port, an outflow port, a valve chamber intervening between the inflow and outflow ports, and a valve hole communicating the valve chamber with the outflow port. This valve housing includes a valve rod inserted exteriorly of the valve housing and into the valve chamber. The valve rod is capable of being axially moved between open and closed positions when a manipulatable handle operatively coupled with the valve rod and positioned external to the valve housing is turned, thereby to adjust the opening of the valve hole.
In the prior art throttle valve assembly, a generally elongated valving member used to close the valve hole when the valve rod is moved to the closed position has a diameter generally smaller than a shank portion of the valve rod which is used to seal the valve chamber.
Also, the valve rod generally used in the prior art throttle valve assembly is integrally formed with an adjustment screw member which is threadingly engaged in the valve housing and is adapted to be rotated by turning the handle.
In view of the foregoing, the prior art throttle valve assembly has been found having the following problems. Namely, assuming that a fluid medium under pressure is supplied to the throttle valve assembly, and particularly when the throttle valve assembly is adjusted to discharge the fluid medium at a relatively low rate, a relatively large axial thrust tending to displace the valve rod outwardly in the axial direction acts on the valve rod by reason of a difference in cross-sectional area between the valving member and the shank portion of the valve rod. This relatively large axial thrust causes a surface pressure to be generated between tooth faces of an internally threaded portion formed in the housing and those of an externally threaded portion formed on the adjustment screw. Because of this surface pressure, the tooth faces of the internally threaded portion in the housing and those of the externally threaded portion on the adjustment screw are frictionally engaged when the handle is turned, imposing a resistance to the turn of the handle. Therefore, the handle becomes hard to turn, requiring an increased turning force to be applied thereto. This tends to hamper a fine flow adjustment.
Also, in the prior art throttle valve assembly, since the valve rod and the adjustment screw member are formed integrally therewith, a relatively highly precise machining is required in order for the assembly of the valve rod and the adjustment screw member to be accurately aligned with the internally threaded portion in the valve housing and/or the valve hole, rendering it difficult to machine the adjustment screw member and the valve rod.